Wow, I'm sorry to hear what you're going through. That's really upsetting the way people treat you. I'm shocked that your trans friends would have that kind of behavior as well; definitely doesn't seem the norm.
Honestly, I've been on T for 10 months now, although I've had some friends calling me "he" for almost two years. Despite the hormones, people still constantly call me "she" and "m'aam" in public. I know they don't realize I'm trans, and I'm not going to correct strangers on it; I figured that once I pass as male, people will just naturally call me that. Until then, I'm just stuck in this transition phase. However, my mom constantly calls me by the wrong name, and talks about me to people outside the house as her "daughter". I know it's been hard on her (she only wants a daughter, not a son; hell, not even a masculine daughter), and yet again, I figure, when I pass all the time, it'll get easier for everyone else.
However, over time, I'm starting to notice the negative toll it's been taking on me mentally. At my work, I'm always called the right name, but since my boss is Chinese, she calls everyone "he" and "she" interchangeably (she mixes a lot of close words up, although her English is actually very good). I'm so used to being referred to as "she" and being misgendered by strangers, that I don't see myself as a man, or any form of it. I really don't see myself as anything at this point. What really brings this to my attention is that when people call me "he" now, I don't even register that these people are talking about me. I always think, "He who?" When I realize they're talking about me, I feel like it's only out of respect, and I know they would never refer to me as that if we were strangers. I feel so low when I have these thoughts, like I'm nothing, nobody. The sudden realization will always disconnect me from my surroundings for a bit, until I can do something to take my mind off of it. I'm constantly keeping busy simply so I don't have time to think about myself and my transition.
The point is, you need to stand up for yourself, and maybe have a heart-felt conversation with these people, before you get too far down the road and realize it's caused you to develop even more issues about yourself. Try to keep from getting overly emotional (emotional outbursts tend to make people think you're just having a moment, and they may not realize the seriousness of the issue in the long-term). Try not to push all the blame onto people, that will also probably rub them the wrong way and make them less likely to listen. Have a cool-tempered, down-to-Earth conversation with them. Explain to them what you're going through, how tough it is on you already, and how their actions are making you feel. Maybe link them to some articles online, or some YouTube videos explaining trans issues. I've found people always start to change their tone when you throw some science their way, and explain that there are biological differences in the way a trans person's brain has developed (trans men usually have brains closer to a cis-male, NOT a female's, and vice versa for trans women). If it might help, just tell them to think of you as their 16-year-old guy friend, and you just haven't hit puberty yet.
If they continue with their behavior, it really might be time to cut them loose. Especially the girlfriend; if you two break up, no matter how close you feel to her, she won't be the last girl you ever date. It may save you some psychological damage in the long run (I'm known for sticking it out in relationships that should have been over almost immediately, for years). I just got out of my first relationship since being "out", and the way he treated me has really messed me up. I'm honestly not sure how long I might be too damaged to date again. Recovery is incredibly difficult, and prevention is your best defense. Of course, always give people time to adjust, but if you just keep putting yourself in the line of abuse, before you know it, years will have gone by, and you may not even know who you are anymore.
Things are always going to be more difficult for trans people, especially if you're pre- hormones or surgery. People won't always accept you, and that sucks. But keep your head high, and realize that everything you're going through will allow you to grow, to be a bigger and better person than those around you. Best of luck with everything, I hope you can find some peace in all your relationships. Always put yourself first.